It is a feature common of all prior-art harvesters of the type described above that, during the felling movement, they are arranged to pivot forwards in the direction towards the trimming knives so that, in the horizontal position, the processor unit is placed above the tree trunk and holds its grasp on the trunk only by means of the trimming members or knives. In order that the tree trunk could be kept in its place, it is required that the trimming members in each group are pressed against each other and, thus, against the tree trunk with a very intensive force, because in this position the drive rolls have vertically directed shafts of rotation and press the tree trunk each of them from its side, as a result of which they cannot contribute to supporting of the tree trunk to a significant extent. This has the consequence that the force of friction that has to be overcome by the drive rolls when feeding the tree trunk during the trimming stage becomes enormously high and results therein that the trimming knives cut unduly deep into the wood in the tree trunk, at the same time as the spikes or equivalent friction-increasing members on the drive rolls also tear deep wounds into the wood in the trunk. Thus, a considerable portion of the topmost layer of wood is lost. Since, as a rule, the trimming members cannot keep a tree trunk in a fully stable position during a trimming process, this also results therein that the cutting of the tree trunks in predetermined lengths does not take place precisely, but major variations occur as a result of the fact that the operation of the measuring means that are used for this purpose is disturbed greatly.